Ultrastructure, calcium accumulation, and contractile response in smooth muscle

Area of Science:

• Muscle physiology
• Cellular biology
• Calcium signaling

Background:

• Extracellular calcium removal affects smooth muscle contractility.
• Guinea pig vas deferens (VD) and taenia coli (TC) exhibit differential mechanical responses to electrical stimulation.
• Intracellular calcium stores are crucial for muscle contraction.

Purpose of the Study:

• To investigate the reasons behind the differing mechanical responsiveness durations of VD and TC after extracellular calcium depletion.
• To compare the intracellular calcium storage capacity and affinity in VD and TC.
• To elucidate the role of intracellular calcium sequestration in smooth muscle contractile differences.

Main Methods:

• Mechanical responsiveness testing of VD and TC following extracellular calcium chelation using [ethylene bis(oxyethylenenitrilo)]tetraacetic acid.
• Comparison of the volume of intracellular storage structures (e.g., sarcoplasmic reticulum, mitochondria) in VD and TC.
• Quantification of 45Ca uptake and distribution in VD and TC via 45Ca autoradiography.

Main Results:

• VD remained mechanically responsive to electrical stimulation for a significantly longer period than TC.
• The volumes of intracellular calcium-sequestering structures were not significantly different between VD and TC.
• VD accumulated approximately twice as much 45Ca as TC, with higher concentrations in surface vesicles, sarcoplasmic reticulum, and mitochondria.

Conclusions:

• The differing mechanical responsiveness of VD and TC is primarily attributed to variations in the affinity of their intracellular calcium storage sites.
• The higher affinity for calcium in VD's storage sites allows for sustained contraction compared to TC.
• These findings highlight the critical role of calcium storage affinity in regulating smooth muscle contractile dynamics.